The invention relates to a control system for a stepless variable speed transmission of power splitting type for use in a vehicle such as automobiles in which the power from a prime mover is transmitted with a controllable speed and wherein part of the power is converted into hydraulic power by means of hydraulic pump motor for controlling a speed ratio in a stepless or continuous manner.
For use with vehicles, hydraulic pump motor employed in a stepless transmission of power splitting type is subject to limitations that a large sized motor cannot be used and that it cannot be operated at a high speed of rotation. In the prior art practice, a mode conversion system is proposed in which a switching of power transmission control means such as clutch or brake is used in combination with a change of speed ratio regions in order to enable the achievement of an extensive range of speed control with a small size hydraulic pump motor which is operated at a low speed. When the mode conversion system is employed, it is necessary to achieve a synchronization between the drive end and the driven end of a clutch that is being engaged in order to avoid undue conditions during the clutch switching operation. A clutch is operated hydraulically, and an engaged clutch will be released simultaneously with the discharge of hydraulic oil while a released clutch cannot be immediately driven into engagement since an increase in the hydraulic pressure does not occur simultaneously with the supply of hydraulic pressure thereto but occurs rather gradually. Consequently, when a switching of clutches requires that one clutch be disengaged while another be engaged, it will be seen that if the hydraulic operations for the disengagement and the engagement take place concurrently, the both clutches may be released for a moment to cause the transmission to assume a neutral condition in which it is free from any load, causing a sudden rise in the speed of rotation of the prime mover, and resulting in a variety of drawbacks. On the other hand, in a conventional control system, the direction in which the piston of a speed ratio control actuator moves is switched simultaneously as the clutches are switched, giving rise to the likelihood that the control actuator may move in the opposite direction before the clutches are completely switched.